Method of Operating a NAND Flash Memory Device

ABSTRACT

Serial NAND flash memory may be provided with the characteristics of continuous read of the memory across page boundaries and from logically contiguous memory locations without wait intervals, while also being clock-compatible with the high performance serial flash NOR (“HPSF-NOR”) memory read commands so that the serial NAND flash memory may be used with controllers designed for HPSF-NOR memory. Serial NAND flash memory having these compatibilities is referred to herein as high-performance serial flash NAND (“SPSF-NAND”) memory. Since devices and systems which use HPSF-NOR memories and controllers often have extreme space limitations, HPSF-NAND may also be provided with the same physical attributes of low pin count and small package size of HPSF-NOR memory for further compatibility. HPSF-NAND memory is particularly suitable for code shadow applications, even while enjoying the low “cost per bit” and low per bit power consumption of a NAND memory array at higher densities.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to digital memory devices, and moreparticularly to NAND flash memory having physical attributes, readcommand clocking characteristics, and/or read output characteristicscompatible with high performance serial NOR flash memory.

2. Description of Related Art

Serial NOR flash memory has become a popular alternative to conventionalparallel NOR flash memory. Serial NOR flash memories offer severaladvantages including; lower pin-count, smaller packages, simpler printedcircuit boards, lower power, comparable performance and reduced deviceand system-level costs. Today, serial NOR flash memories are offered indensities from 512 Kilobit to 1 Gigabit, and employ the popular SerialPeripheral Interface (“SPI”).

Single-bit SPI uses four pins for transferring commands, address anddata to, and data from, the serial flash memory, namely: Chip Select(/CS), Clock (CLK), Data In (DI) and Data Out (DO). Multi-bit SPI whichincludes Dual SPI, Quad SPI and Quad Peripheral Interface (“QPI”) usethe same four pins but reconfigurable to transfer more serial data perclock cycle. Dual SPI changes the DI and DO pins to bi-directional DIO(Input/Output) pins. Quad SPI also changes the DI and DO pins to DIOpins, and adds two additional DIO pins for a total of four DIO pins, fora total of six pins when /CS and CLK are considered. QPI has four DIOpins like Quad SPI, but allows for full quad (four DIO) operation evenfor initial commands. These multi-bit SPI variants, combined withincreasing clock speeds, allow Serial NOR Flash to be used for fast codeshadowing to Random Access Memory (“RAM”); see, e.g., U.S. Pat. No.7,558,900 issued Jul. 7, 2009 to Jigour et al.

Code shadowing tends to be performed in the following manner. Duringsystem boot-up, all or a portion of the non-volatile data is transferredfrom the serial NOR flash into system random access memory (“RAM”). Codeshadowing may also be done dynamically after system boot, where asmaller RAM may be time-shared as needed by dynamically shadowingportions of the larger serial NOR flash memory.

Since system boot-up time is directly related to how fast the code canbe shadowed, the higher the performance of the serial NOR flash, thefaster the system can boot. Typically, a single SPI read command isissued with a starting address, and then data is continuously clockedout until all needed code is transfer to RAM. Today's serial NOR flashmemory can achieve continuous read transfer rates in excess of 50megabytes/second when using the quad SPI interface at 104 MHz.Applications like digital TVs, set-top boxes, personal computers, DVDplayers, networking equipment and automotive displays are examples ofapplications that benefit from code shadowing with high speed serial NORflash memory. Application specific controllers commonly design basicserial NOR flash SPI read commands into the hardware circuitry(“hardcoded”) so that upon power-up, all or a portion of the data can bequickly loaded into RAM for operation. The 03 hex Read command, forexample, is typically hardcoded.

At densities of 256 Megabits and higher, the cost of serial NOR flashmemory approaches and exceeds the cost of single level cell (“SLC”) NANDflash memory in densities of 512 Megabits and higher. The cost versusdensity advantage of SLC NAND flash memory is largely due to theinherently smaller memory cell size used in SLC NAND flash technology,which makes the cost to manufacture highly dense NAND flash memory muchlower than NOR flash memory. Unfortunately, commonly used SLC NAND flashmemory has architectural, performance and bad block limitations thatmake it difficult to support the high speed code shadow applications forwhich serial NOR flash memory is well suited.

Serial NOR flash memory allows data to be clocked out of the device froma specified starting address (such as address 0) in a continuous andsequential fashion, without any delay time between clocks or any need towait and check whether the device is ready or busy. In contrast, NANDflash memory has relatively long access times per page, typically tRD=25uS for a 2048+64 byte page. Once the page has been accessed, the data isclocked out sequentially and quickly, typically 25 nS per byte, but thenanother tRD is incurred for the next page access. Some NAND flash memoryprovide a cache read feature that allows the next page to be accessedwhile data from the previous page is being clock out. However, thisoperation still uses a Ready/Busy check to confirm that the NAND flashmemory is ready to proceed, which results in slower code shadowperformance.

While today's NAND flash memory can ideally achieve read transfer ratesof 25 to 35 megabytes/second, this does not take into account time forhandling error correction code (“ECC”) processing and bad blockmanagement. These activities can further reduce the transfer rate byhalf, and result in performance significantly lower than serial NORflash memory.

NAND flash memory allows for a certain percentage, typically 2%, of theblocks (typically 64 pages per block, 128 kilobytes+4 kilobytes) to bebad and not usable for the application. Typically, these bad blocks canbe located anywhere in the memory array, and so are tagged so that theycan be identified and not used. Some NAND flash memories guarantee onlythe first block to be good. As a result, standard sequential andcontinuous code shadowing is unreliable since the next block accessedmay be bad. In contrast, serial NOR flash memory offers 100% good memorycells over the entire addressable memory range.

The data integrity of NOR flash memory is also better than NAND flashmemory. In fact, external application ECC software or internal on-chipECC circuitry is typically used with SLC NAND flash memory to locate andcorrect single bit, or in some cases, multi-bit errors. While NAND flashmemory with on-chip ECC tend to perform faster than external ECC,undesirable delays of up to 100 uS per page read must be taken intoconsideration.

Serial NOR flash memory is available with the 4 to 6 active pin SPIinterface and in small space efficient packages such as the 8-contactWSON, the 24-ball BGA, and the 8-pin and 16-pin SOIC. In contrast,ordinary parallel NAND flash memory typically employs 14 to 22 activepins housed in a relatively large 48-pin TSOP or 63-Ball BGA packagethat consumes up to twice the printed circuit board space of a serialNOR flash memory; see, for example, SK Hynix Inc., I Gbit (128M×8bit/64M×16 bit) NAND Flash Memory, Rev. 1.1, November 2005; MicronTechnology, Inc., 1 Gb NAND Flash Memory, Rev. E, 2006. Ordinary serialNAND flash memory have been introduced with the SPI interface; see, forexample, Micron Technology, Inc., Get More for Less in Your EmbeddedDesigns with Serial NAND Flash, Jul. 28, 2009, but such ordinary serialNAND flash memory tends to be housed in larger packages such as the63-ball BGA, and have the same architectural, performance and bad blocklimitations that ordinary NAND flash memory has. Additionally theseserial NAND flash memories do not offer command compatibility with theserial NOR flash memories on the market; see, e.g., Winbond ElectronicsCorporation, W25Q64CV SpiFlash 3V 64M-Bit Serial Flash Memory with Dualand Quad SPI, Revision F, May 7, 2012; Winbond Electronics Corporation,W25Q128FV SpiFlash 3V 128M-Bit Serial Flash Memory with Dual/Quad SPI &QPI, Revision D, Oct. 1, 2012.

BRIEF SUMMARY OF THE INVENTION

While serial NOR flash memory is a popular solution for code shadowingapplications, the cost structure at higher densities is not favorablewhen compared with the cost structure for NAND flash memory. Whileserial NAND flash memory has been introduced, it has architectural,performance and bad block limitations that compromise its usefulness forhigh performance sequential and continuous code shadowing, and does notoffer command compatibility with serial NOR flash memory.

What is needed is a serial NAND flash memory that retains its costadvantage over serial NOR flash memory at high density, yet can acceptserial NOR flash memory compatible read commands for use with existingserial NOR controllers, and is package-compatible with and hasarchitectural and performance characteristics comparable to serial NORflash memory to enable continuous read without delays throughout thememory for code shadowing operations.

One or more of these and other advantages may be achieved by the variousembodiments of the present invention. One embodiment of the presentinvention is a serial NAND flash memory comprising: a package selectedfrom a group consisting of an 8-pin WSON package, a 24-pin FBGA package,an 8-pin SOIC package, and a 16-pin SOIC package, wherein at least someof the pins of the package are active pins of an SPI interface; a NANDflash memory array contained in the package; a page buffer contained inthe package and coupled to the NAND flash memory array; and controllogic contained in the package and coupled to the NAND flash array andthe page buffer for providing, in response to a read command, dataoutput from the NAND flash memory device to at least one of the pins ofthe active SPI interface via the page buffer.

Another embodiment of the present invention is a serial NAND flashmemory device comprising: a package having a footprint of 48 millimeterssquared or less, and an active SPI interface of from four to six pins; aNAND flash memory array contained in the package; a page buffercontained in the package and coupled to the NAND flash memory array; andcontrol logic contained in the package and coupled to the NAND flasharray and the page buffer for providing, from the NAND flash memorydevice to at least one of the pins of the active SPI interface via thepage buffer, in response to a read command, a continuous data outputacross page boundaries and from logically contiguous memory locationswithout wait intervals.

Another embodiment of the present invention is a serial NAND flashmemory device comprising: an interface; a NAND flash memory array; apage buffer coupled to the NAND flash memory array; a control logiccoupled to the NAND flash memory array and the page buffer forproviding, from the NAND flash memory device to the interface via thepage buffer, in response to a read command, a continuous data outputacross page boundaries and from logically contiguous memory locationswithout wait intervals; and a power-up detector for initiating loadingof a default page of the NAND flash memory array to the page buffer uponpower-up.

Another embodiment of the present invention is a method of operating aNAND flash memory device comprising: receiving a read command whichcorresponds to a high-performance serial flash NOR (“HPSF-NOR”) readcommand and is clock-compatible therewith; and providing, from the NANDflash memory device in response to the read command receiving step, acontinuous data output across page boundaries and from logicallycontiguous memory locations without wait intervals.

Another embodiment of the present invention is a method of operating amemory having a NAND flash memory array and a page buffer associatedwith the NAND flash memory array, comprising: selecting a page of theNAND flash memory array; storing data from the selected page in the pagebuffer; performing ECC computations on the data in the page buffer;outputting the data from the page buffer; and repeating the pageselecting, data storing, ECC computation performing, and data outputtingsteps so that data output is continuous across page boundaries and fromlogically contiguous memory locations without wait intervals; whereinthe page selecting step initially comprises selecting a default page inthe NAND flash memory array, and subsequently comprises selectingsuccessive sequential pages of the NAND flash memory array; wherein thepage selecting, data storing, and ECC computation performing steps areinitially performed automatically during power-up of the flash memory,and are subsequently performed in response to a read command; andwherein the data outputting step is performed in response to the readcommand.

Another embodiment of the present invention is a method of operating aNAND flash memory comprising: selecting a default page of a NAND flashmemory array of the NAND flash memory during power-up thereof; storingdata from the default page of the NAND flash memory array in a pagebuffer during power-up of the NAND flash memory; performing ECCcomputations on the data stored in the page buffer after the storingstep; receiving a read command; and providing, from the NAND flashmemory via the page buffer and in response to the read command receivingstep, a continuous data output across page boundaries and from logicallycontiguous memory locations without wait intervals.

Another embodiment of the present invention is a method of powering upflash memory having a NAND flash memory array and a page bufferassociated with the NAND flash memory array, comprising: setting theflash memory in a continuous read mode or a buffer read mode;transferring a page of data from a default page of the NAND flash memoryarray to the page buffer; ECC processing the default page of data in thepage buffer to provide an ECC processed default page of data; after thedata storing step and the ECC processing step, receiving a read command;and when the flash memory is in the continuous read mode, outputtingfrom the flash memory in response to the read command receiving step, acontinuous data output across page boundaries and from logicallycontiguous memory locations without wait intervals, beginning with theECC processed default page of data in the page buffer; and when theflash memory is in the buffer read mode, outputting from the flashmemory in response to the read command receiving step, a data outputlimited to data in the page buffer.

Another embodiment of the present invention is a method of operating amemory having a NAND flash memory array and a page buffer coupled to theNAND flash memory array, comprising: receiving a continuous read commandcomprising a command code and a starting address; and providing, fromthe NAND flash memory array via the page buffer and in response to theread command receiving step, a continuous data output across pageboundaries and from logically contiguous memory locations without waitintervals; wherein the providing step begins with column 00 of the pagebuffer regardless of the starting address.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a timing diagram for the EBh command for a serial NOR flashmemory.

FIG. 2 is a timing diagram for the EBh command for an operation mode ofa serial NAND flash memory.

FIG. 3 is a flowchart showing the operations of a serial NAND flashmemory during power-up and in use.

FIG. 4 is a schematic functional block diagram of a NAND flash memory.

FIG. 5 is a flowchart of the process of powering up a serial NAND flashmemory.

FIG. 6 is a functional block diagram of the functioning of a bufferhaving a data register and a cache register during the power-up processof FIG. 5.

FIG. 7 is a flowchart of the process of operating a serial NAND flashmemory in a continuous read mode.

FIG. 8 is a functional block diagram of the functioning of a bufferhaving a data register and a cache register during the continuous readmode process of FIG. 7.

DETAILED DESCRIPTION OF THE INVENTION, INCLUDING THE BEST MODE

High Performance serial flash memory which uses NOR memory arraytechnology (“HPSF-NOR”) such as types W25Q64CV and W25Q128FV availablefrom Winbond Electronics Corporation of San Jose, Calif., USA, anddescribed in, for example, various data sheets such as WinbondElectronics Corporation, W25Q64CV SpiFlash 3V 64M-Bit Serial FlashMemory with Dual and Quad SPI, Revision F, May 7, 2012, and WinbondElectronics Corporation, W25Q128FV SpiFlash 3V 128M-Bit Serial FlashMemory with Dual/Quad SPI & QPI, Revision D, Oct. 1, 2012, which herebyare incorporated herein in their entirety by reference thereto, hascertain read characteristics which render the HPSF-NOR memoryparticularly suitable for applications involving code shadowing(transference of executable code or data such as parameters, text,images, audio, and so forth), which are common in such electronicdevices and systems as digital televisions, set-top boxes, personalcomputers, DVD players, networking equipment and automotive displays.These read characteristics are a continuous read of the memory acrosspage boundaries and from logically contiguous memory locations withoutwait intervals. The term “continuous read across page boundaries” asused herein refers to a read which proceeds sequentially through thememory in response to a single read command, without need for anyadditional read commands or addresses at page boundaries. The term “waitinterval” as used herein refers to any gap in the data output stream dueto a page read (“tRD”), ECC processing, bad block management, ready/busystatus checking, or any other architecture-required orapplication-required delay. Moreover, physical space commonly is at apremium in such electronic devices and systems, and the HPSF-NOR memoryalso has certain physical attributes such as low pin count and smallpackage size which render the HPSF-NOR memory particularly suitable forsuch electronic devices and systems. Application specific controllersare available that use HPSF-NOR memory for system boot and code shadowapplication and are designed to issue commands which are compliant withthe command input requirements of the HPSF-NOR memory, including readcommand clocking requirements. Application specific memory controllersalso may design one or more basic HPSF-NOR read commands into thehardware circuitry (“hardcoded”), so that upon power-up and even duringnormal operation, all or a large portion of the stored data can bequickly loaded into RAM for operation. The hardcoded command(s) mayinclude the Read command 03h and the Fast Read command 0Bh, and may alsoinclude any of the available Multi-I/O Read commands. The hardcodedstarting address for the controller is commonly address 0, but otheraddress locations may be used if desired.

As described herein, serial NAND flash memory may be made compatiblewith the characteristics of a continuous read of the memory across pageboundaries and from logically contiguous memory locations without waitintervals, while also being clock-compatible with the HPSF-NOR readcommands so that the serial NAND flash memory can accept HPSF-NOR readcommands and be used with controllers designed for HPSF-NOR memory.Serial NAND flash memory having these compatibilities is referred toherein as high-performance serial flash NAND memory, or “HPSF-NAND.”Since devices and systems which use HPSF-NOR memories and controllersoften have extreme space limitations, HPSF-NAND may also be providedwith the same physical attributes of low pin count and small packagesize of HPSF-NOR memory for further compatibility, even in somecircumstances to the extent that a HPSF-NAND memory may have anidentical footprint to and the identical pinout of a HPSF-NOR memory andmay be substituted for it in a printed circuit board without any changeto the printed circuit board. Although compatible with HPSF-NORcharacteristics, HPSF-NAND enjoys the low “cost per bit” and low per bitpower consumption of a NAND memory array in its optimal density range of512 Mb and greater.

HPSF-NAND memory may be made compatible with many characteristics ofHPSF-NOR memory, including: (1) the multi-I/O SPI/QPI interface; (2)small low pin count package types (as small as 8×6 mm at densities of256 Mb and higher) such as, for example, the 8-contact WSON, 16-pinSOIC, and the 24-ball BGA type packages, with the flexibility of usinglarger packages such as VBGA-63 typically used with ordinary paralleland ordinary serial NAND flash memory; (3) high clock frequencyoperation (illustratively 104 MHz) for high transfer rates(illustratively 50 MB/Sec); (4) continuous read across page boundarieswithout wait intervals, for fast code shadowing applications; (5)logically contiguous addressable good memory through bad blockmanagement which is transparent to the external systems and which iswithout adverse impact on the speed and continuity of the output; and(6) an output starting address of zero or alternatively any otheraddress within the user-addressable space of the memory array via auser-specified or manufacturer-specified value. Advantageously,HPSF-NAND as described herein may be used with existing HPSF-NOR memorycontrollers and systems, yet may have a cost that is comparable toordinary NAND flash memory.

Table 1 provides a summary comparison of various features of ordinaryparallel SLC NAND memory, ordinary serial NAND memory, and HPSF-NORmemory with the HPSF-NAND memory described herein.

TABLE 1 ORDINARY ORDINARY PARALLEL SERIAL FEATURE NAND NAND HPSF-NORHPSF-NAND Optimal ≧512 Mb ≧512 Mb ≦256 Mb ≧512 Mb Density Range (Costper bit) Continuous NO NO YES YES Read Without (1) Read 1 (1) Read 1 (1)No (1) No Wait Intervals page at a time page at a time Ready/BusyReady/Busy (suitable for (2) Wait for (2) Wait for between pages betweenpages code shadow) tRD, ECC, tRD, ECC, (2) ECC not (Buried tRD) and/orand/or needed (2) Buried ECC Ready/Busy Ready/Busy time Logically NO NOYES YES contiguous Bad Blocks Bad Blocks Bad Blocks Addressable disallowdisallow mapped out to Good Memory contiguous contiguous end of user-memory memory addressable memory Clock- NO NO YES YES Compatible SerialFlash Read Commands Clock 40 MHz 50 MHz 104 MHz 104 MHz FrequencyTransfer Rate 20 MB/Sec-50 MB/Sec About 50 MB/Sec 50 MB/Sec 15 MB/SecInterface ONFI Multi I/O SPI Multi I/O SPI Multi I/O SPI Package TypeTSOP-40 VBGA-63 WSON-8 WSON-8 BGA-63 FBGA-24 FBGA-24 SOIC-8 SOIC-8SOIC-16 SOIC-16 VBGA-63 Smallest 9 × 11 mm 9 × 11 mm 8 × 6 mm 8 × 6 mmPackage (256 Mb or higher density) Typical 14/22 4/6 4/6 4/6 Number ofInterface Pins Compatible NO NO YES YES Pin-Outs and Packages withSerial Flash

Clock-Compatible Serial Flash Read Commands

HPSF-NOR memory devices such as types W25Q64CV and W25Q128FV availablefrom Winbond Electronics Corporation of San Jose, Calif., USA, support avariety of SPI commands, including various read commands. Severalillustrative read commands which are supported in SPI mode, for example,are shown in Table 2.

TABLE 2 Byte1 Command OpCode Byte2 Byte3 Byte4 Byte5 Byte6 Byte7 Byte8READ A23-A16 A15-A8 A7-A0 D7-D0 D7-D0 D7-D0 D7-D0 03h FAST READ A23-A16A15-A8 A7-A0 DUMMY D7-D0 D7-D0 D7-D0 0Bh FAST READ A23-A16 A15-A8 A7-A0DUMMY D7-D0 D7-D0 D7-D0 DUAL OUTPUT 3Bh FAST READ A23-A16 A15-A8 A7-A0DUMMY D7-D0 D7-D0 D7-D0 QUAD OUTPUT 6Bh FAST READ A23-A16 A15-A8 A7-A0M7-M0 D7-D0 D7-D0 D7-D0 DUAL I/O BBh FAST READ A23-A16 A15-A8 A7-A0M7-M0 DUMMY DUMMY D7-D0 QUAD I/O EBh

The Read command 03h allows one or more data bytes to be sequentiallyread from the page buffer 338. Byte2, Byte3 and Byte4 contain a 24-bitaddress for reading out a data byte from the addressed memory location.The 03h Read command plus address uses a total of 32 clocks before datais made available. The address is automatically incremented to the nexthigher address after each byte of data is shifted out, allowing for acontinuous stream of data for as long as the clock continues. Thecommand is completed by driving /CS high. The Fast Read command 0Bh issimilar to the Read command except that it can operate at the highestspecified frequency due to the inclusion of 8 dummy clocks after the24-bit address. The dummy clocks allow the devices internal circuitsadditional time for setting up the initial address. The 0Bh Fast Readcommand plus address uses a total of 40 clocks before data is madeavailable. The Fast Read Dual Output command 3Bh is similar to the 0BhFast Read command except that data is output on two pins, IO₀ and IO₁,instead of one pin. The Fast Read Quad Output command 6Bh is alsosimilar to the 0Bh Fast Read command except that data is output on fourpins, IO₀, IO₁, IO₂ and IO₃. The Fast Read Dual I/O command BBh and theFast Read Quad I/O command EBh are similar to the 3Bh Fast Read DualOutput command and the 6Bh Fast Read Quad Output command, but with thecapability to input the Address bits (A23-A0) either two or four bitsper clock, respectively. The Fast Read Dual I/O and the Fast Read QuadI/O commands also include 8 mode bits M7-M0 after the 24-bit address.

Table 3 lists a number of illustrative read commands for the HPSF-NANDmemory which correspond to and are clock-compatible with—that is, havethe same “address/dummy” cycle numbers or clocks as—the read commands ofTable 2 for the HPSF-NOR memory.

TABLE 3 Byte1 Command OpCode Byte2 Byte3 Byte4 Byte5 Byte6 Byte7 Byte8READ DUMMY DUMMY DUMMY D7-D0 D7-D0 D7-D0 D7-D0 03h FAST READ DUMMY DUMMYDUMMY DUMMY D7-D0 D7-D0 D7-D0 0Bh FAST READ DUMMY DUMMY DUMMY DUMMYD7-D0 D7-D0 D7-D0 DUAL OUTPUT 3Bh FAST READ DUMMY DUMMY DUMMY DUMMYD7-D0 D7-D0 D7-D0 QUAD OUTPUT 6Bh FAST READ DUMMY DUMMY DUMMY DUMMYD7-D0 D7-D0 D7-D0 DUAL I/O BBh FAST READ DUMMY DUMMY DUMMY DUMMY DUMMYDUMMY D7-D0 QUAD I/O EBh

As will be appreciated from a comparison of Table 2 and Table 3, dummybytes are used in place of the address and mode bytes, so that theHPSF-NAND read commands of Table 3 are clock-compatible with theHPSF-NOR read commands of Table 2 to which they correspond. TheHPSF-NAND read commands of Table 3 perform the same read functions asthe HPSF-NOR read commands of Table 2 to which they correspond, andallow for the same continuous stream of data for as long as the clockcontinues. This ensures that, regardless of the address the controllerprovides while clocking in a HPSF-NOR Read command, data will come outof the HPSF-NAND memory and is continuously clocked out in the same wayas HPSF-NOR memory operates. They differ in that for the HPSF-NAND readcommands, reading starts from Column 00 of whatever page resides in thememory buffer, whereas for the HPSF-NOR read commands, reading startsfrom any array location specified by A[23:0].

If desired, HPSF-NAND read commands may specify the address location,and the HPSF-NAND memory may begin the read at the column specified inthe address. Various design considerations for this implementationinclude the system clock (relatively slow), speed of certain internaloperations such as ECC (relatively fast), and allowable startinglocations in the page buffer, so that sufficient time may be providedfor the subsequent operation. Starting the read at Column 00 relaxessuch design constraints without sacrificing the usefulness of theHPSF-NAND memory for code shadowing.

For memory densities of 256 Mb and greater, the standard SPI readcommands use an additional byte of address. Table 4 shows severalillustrative read commands for large density HPSF-NOR memory, and Table5 shows corresponding read commands for large density HPSF-NAND memory.

TABLE 4 Byte1 Command OpCode Byte2 Byte3 Byte4 Byte5 Byte6 Byte7 Byte8Byte9 READ A31-A24 A23-A16 A15-A8 A7-A0 D7-D0 D7-D0 D7-D0 D7-D0 03h FASTREAD A31-A24 A23-A16 A15-A8 A7-A0 DMY D7-D0 D7-D0 D7-D0 0Bh FAST READA31-A24 A23-A16 A15-A8 A7-A0 DMY D7-D0 D7-D0 D7-D0 DUAL OUTPUT 3Bh FASTREAD A31-A24 A23-A16 A15-A8 A7-A0 DMY D7-D0 D7-D0 D7-D0 QUAD OUTPUT 6BhFAST READ A31-A24 A23-A16 A15-A8 A7-A0 M7-M0 D7-D0 D7-D0 D7-D0 DUAL I/OBBh FAST READ A31-A24 A23-A16 A15-A8 A7-A0 M7-M0 DMY DMY D7-D0 QUAD I/OEBh

TABLE 5 Byte 1 Command OpCode Byte2 Byte3 Byte4 Byte5 Byte6 Byte7 Byte8Byte9 READ DMY DMY DMY DMY D7-D0 D7-D0 D7-D0 D7-D0 03h FAST READ DMY DMYDMY DMY DMY D7-D0 D7-D0 D7-D0 0Bh FAST READ DMY DMY DMY DMY DMY D7-D0D7-D0 D7-D0 DUAL OUTPUT 3Bh FAST READ DMY DMY DMY DMY DMY D7-D0 D7-D0D7-D0 QUAD OUTPUT 6Bh FAST READ DMY DMY DMY DMY DMY D7-D0 D7-D0 D7-D0DUAL I/O BBh FAST READ DMY DMY DMY DMY DMY DMY DMY D7-D0 QUAD I/O EBh

FIG. 1 is an illustrative timing diagram of the Fast Read Quad I/Ocommand for the HPSF-NOR memory, and FIG. 2 is an illustrative timingdiagram of the Fast Read Quad I/O command for the HPSF-NAND memory.These figures may be compared to better understand theclock-compatibility of the two commands. The EBh command is receivedduring the first eight clocks 0 through 7 for both memory types. Duringthe next eight clocks 8 through 15, the 24-bit address is received overpins IO₀, IO₁, IO₂ and IO₃ of the HPSF-NOR memory, while the states ofthe pins IO₀, IO₁, IO₂ and IO₃ are ignored by the HPSF-NAND memory. Thenext four clocks 16 through 19 are dummy clocks corresponding to twodummy bytes for both memory types, and clock 20 begins continuous dataoutput on the pins IO₀, IO₁, IO₂ and IO₃ of both memory types.

Mode Selection

While the HPSF-NOR continuous read starting from Column 00 of the pageresident in the memory buffer is particularly advantageous for codeshadow operations, need may arise for operation in other modes. Oneexample is when a read is desired which begins at a column address otherthan 00. While this capability may be obtained by modifying theHPSF-NAND memory continuous read feature to use a starting address,another approach is to provide mode switching. Mode switching may alsobe used to switch to a mode which does not support the continuous readacross page boundaries without wait intervals. Another example is when aHPSF-NAND memory is desired to power up in the HPSF mode for codeshadowing to RAM, but then operate in another mode compliant with a NANDmemory standards such as the Open NAND Flash Interface (“ONFI”) andordinary serial NAND flash memory. Alternatively, the HPSF-NAND memorymay power up by default in the other mode, and be switchable to thecontinuous read mode.

One may desire, for example, for a HPSF-NAND memory to be switchablebetween a continuous read mode and a buffer read (single page read)mode. Table 6 shows the timing for an illustrative Read command 03h foran ordinary serial NAND flash memory, which operates in buffer readmode. The timings for other commands such as the Fast Read command 0Bh,the Fast Read Dual Output command 3Bh, the Fast Read Quad Output command6Bh, the Fast Read Dual I/O command BBh, and the Fast Read Quad I/Ocommand EBh may be established for the ordinary serial NAND flash memoryin a similar manner as for the Read command 03h. The timing for theordinary serial NAND flash memory uses two bytes for column addressC[15:0] and one dummy byte following the column address bytes. Not shownin Table 6 due to space limitations is that the data streamed inresponse to the Read command for the ordinary serial NAND flash memoryeither terminates at the end of the buffer or wraps around to the startof the buffer, until terminated by a /CS transition. If one desires toread additional pages, a further command may be issued, but this actioncomes at the expense of delays due to Ready/Busy checking and the timeneeded to read a page from the NAND memory array.

TABLE 6 Byte1 Command OpCode Byte2 Byte3 Byte4 Byte5 Byte6 Byte7 Byte8ORDINARY C15-C8 C7-C0 DUMMY D7-D0 D7-D0 D7-D0 D7-D0 SERIAL NAND FLASHREAD 03 h

FIG. 3 shows an illustrative process of mode selection for a HPSF-NANDmemory during and after power-up, in which the HPSF-NAND memory startsup by default in the continuous read mode in which various read commandshave the same syntax as HPSF-NOR commands and are command clockingcompatible and continuous read output compatible therewith, andthereafter is switchable to an ordinary serial NAND flash memory bufferread mode or to the continuous page read mode as desired. Upon power-up(block 200), a buffer mode flag BUF is set to 0 by default, andautomatic loading of a default page into the buffer of the serial NANDflash memory is begun (block 210). Illustratively the default page ispage 0, although any page may be established in the serial NAND flashmemory as the default page. An illustrative technique for setting thedefault page is to store the default page address in a configurationregister (not shown), access to which may be restricted to themanufacturer or open to the OEM or user. At the start of loading of thedefault page, a BUSY flag is set to 1 to prevent the default page loadfrom being interrupted by another command. However, certain commands maybe allowed to be functional during the busy period without affecting thebuffer mode flag BUF, such as, for example, the Get Feature or ReadStatus Register command 05h or 0Fh to check the BUSY flag, and the JEDECID command 9Fh to check device identification (block 220). Othercommands may be allowed to change the buffer mode flag to 1 during thebusy period, such as, for example, the Device Reset command FFh may beused (block 230 yes and block 240); otherwise the buffer mode remains 0for (illustratively) all other commands (block 230 no).

Alternatively or additionally, the buffer mode flag BUF may be changedto 1 if the first command (not shown) after the BUSY flag returns to 0is the Device Reset command FFh. Any other command leaves BUF=0 and isexecuted in the normal manner.

A read command received after the busy period is executed in a mannerwhich depends on the value of BUF. If BUF=0, any column address data inthe command is ignored and the read operation (block 250(0)) starts from0x00h column and continues through successive pages until terminatedwhen /CS is asserted high. If BUF=1, the read operation (block 250(1))begins at the column address [11:0] specified in the command andterminates when the end of the buffer is read or when /CS is assertedhigh.

Thereafter, the memory may be operated as desired using variousmiscellaneous operations (block 260), program and erase operations(block 270), page load operations (block 280), and read operations(block 290(0) or block 290(1) depending on the value of BUF. The buffermode flag BUF may be set at either 0 or 1 as many times as desiredduring normal operation by writing to BUF in the status register usingthe Set Feature or Write Status Register command 1 Fh or 01 h. TheDevice Reset command FFh may be used to interrupt any on-going internaloperations. The page load operation (block 280) may be initiated usingthe Page Data Read command 13h, which allows a full page of data to beread from the NAND flash array 342 (FIG. 4) into the page buffer 338(FIG. 4). Byte2 contains dummy bits for timing, and Byte3 and Byte4contain the page address. A read command received after a page load ofpage XX (block 280) is executed in a manner which depends on the valueof BUF. If BUF=0, any column address data in the command is ignored andthe read operation (block 290(0)) starts from 0x00h column and continuesthrough successive pages until terminated when /CS is asserted high. IfBUF=1, the read operation (block 290(1)) begins at the column address[11:0] specified in the command and terminates when the end of thebuffer is read or when /CS is asserted high.

Serial NAND Flash Memory Architecture

FIG. 4 is a schematic functional block diagram of an illustrativeHPSF-NAND memory 320 which is capable of providing a continuous readacross page boundaries and from logically contiguous memory locationswithout wait intervals. The serial NAND flash memory 320 includes a NANDflash array 340 and associated page buffer 338. The NAND flash array 340includes word (row) lines and bit (column) lines, and is organized intoa user-addressable area 342, a redundant block area 344, and a LUTinformation block 346. Any desired flash memory cell technology may beused for the flash memory cells of the NAND flash array 340. The serialNAND flash memory 320 may include various other circuits to supportmemory programming, erase and read, such as row decoder 334, columndecoder 336, I/O control 322, status register(s) 323, continuous pageread (“CPR”) address register(s) 324, command register 325, addressregister 326, a LUT register 327, control logic 330, CPR bad block logic331, a CPR bad block register 332, and high voltage generators 333. Therow decoder 334 selects rows of the user-addressable area 342 under usercontrol as well as, in some implementations, under internal control; andselects rows of the redundant block area 344 and LUT information block346 under internal control. Power is supplied (not shown) throughout thecircuits of the HPSF-NAND memory 320 by power lines VCC and GND. Whilethe NAND flash memory 320 may be packaged in any desired manner and mayhave any type of interface, including ordinary NAND flash memoryinterfaces, the control logic 330 of FIG. 4 illustratively implementsthe SPI/QPI protocol, including the multi-IO SPI interface. Additionaldetail on the SPI/QPI interface and on the various circuits of thememory may be found in U.S. Pat. No. 7,558,900 issued Jul. 7, 2009 toJigour et al., and in a publication by Winbond Electronics Corporation,W25Q64DW: SpiFlash 1.8V 64M-Bit Serial Flash Memory with Dual/Quad SPI &QPI: Preliminary Revision C, Hsinchu, Taiwan, R.O.C., Jan. 13, 2011,which hereby are incorporated herein in their entirety by referencethereto.

If mode switching is desired, a buffer mode flag BUF 347 may beprovided. The buffer mode flag 347 may be provided as a bit of thestatus register(s) 323 if desired. A power-up detector 335 is providedin the control logic 330 to initiate the setting of a particular modeand the loading of a default page upon power-up.

The page buffer 338 illustratively includes a one-page data register(not shown), a one-page cache register (not shown), and one page oftransmission gates (not shown) for copying data from the data registerto the cache register. Any suitable latch or memory technology may beused for the data register and the cache register, and any suitablegating technology may be used for the transmission gates. The dataregister and the cache register may be organized in any desired numberof respective portions by, for example, the manner in which thetransmission gates are wired and operated to control transmission ofdata. Illustratively, the data register and the cache register may beorganized in two respective portions each, and operated in alternationby using respective groups of transmission gates controlled byrespective control lines. The data register and the cache register ofthe page buffer 338 may be operated in a conventional manner by applyingthe same control signal to respective transmission gate control lines,or may be operated in alternation by applying suitable timed controlsignals to the transmission gate control lines. Illustratively in a twoportion implementation in which a page is 2K Bytes, a half-page (1K) oftransmission gates may be controlled by one control line and the otherhalf-page (1K) of transmission gates may be controlled by anothercontrol line, thereby organizing the data register and the cacheregister in two half-page (1K) portions. Because of the operation of twoportions in alternation, a two-portion implementation of the page buffer338 may be referred to as a “ping pong” buffer. An ECC circuit (notshow) may be provided to perform ECC computations on the contents of thecache register depending on the status of an ECC-E flag 348. The ECC-Eflag 348 may be provided as a bit of the status register(s) 323 ifdesired. Additional detail on the page buffer 338, the ECC circuit, andtheir operations may be found in U.S. patent application Ser. No.13/464,535 filed May 4, 2012 (Gupta et al., Method and Apparatus forReading NAND Flash Memory), which hereby is incorporated herein in itsentirety by reference thereto. Continuous page read as described hereinis referred to as “modified continuous page read” in the aforementionedpatent application. This manner of organizing the data register andcache register into portions and performing ECC on the portions isillustrative, and other techniques may be used if desired.

While the NAND flash memory 320 is organized and operated to perform avariety of read operations including continuous page read operations andon-chip ECC in a single-plane NAND Architecture, this architecture isillustrative and variations thereof are contemplated. It will beappreciated that the example of a 2 KB Page size and a specific blocksize are illustrative and may be different if desired. Moreover, thespecific size reference is not to be taken literally, since the actualpage size may vary depending on design factors; for example, the termmay include a 2,048 Byte main area plus an additional 64 Byte sparearea, where the spare area is used for storing ECC and other informationsuch as meta data. In the same way, the term 1 KB may refer to a 1,024Byte main area and a 32 Byte spare area. While the description herein isbased upon a single-plane architecture for clarity, the teachings setforth herein are equally applicable to multi-plane architectures. Whenmultiple physical planes are used, they may share one or more word-linesso that the memory system may service multiple I/O requestssimultaneously. Each plane provides a page of data and includes acorresponding data register of one page size and a corresponding cacheregister of one page size. The techniques described herein may beapplied to each plane separately such that each data register and cacheregister is organized in multiple portions, or may be applied tomultiple planes such that each data register and cache register isitself one portion of a multiple page data register and cache register.

FIG. 4 also shows control signals /CS, CLK, DI, DO, /WP and /HOLD whichare for the SPI interface. The standard SPI flash interface provides /CS(chip select—complement), CLK (clock), DI (serial data-in), and DO(serial data-out) signals, along with optional signals /WP (writeprotect—complement) and /HOLD (hold—complement). While the 1-bit serialdata bus (data-in through DI and data-out through DO) in the standardSPI interface provides a simple interface and compatibility with manycontrollers which boot up in single SPI mode, it is limited in achievinghigher read thru-put. A multi-bit SPI interface therefore evolved toadditionally support dual (2-bit interface) and/or quad (4-bitinterface) for increased read thru-put. FIG. 4 also shows additionaldata bus signals for Dual SPI and Quad SPI operation, i.e. I/O(0),I/O(1), I/O(2), and I/O(3), by selectively redefining the function offour pins. In one illustrative version of the Quad SPI read operation(other versions may be envisioned), the appropriate read command may begiven with 1-bit standard SPI interface through I/O(0), but subsequentinterface for address and data-out may be Quad based (i.e. 4-bit databus). The Quad SPI read operation can output 4-bits of data in a clockcycle as compared to output 1-bit of data in standard SPI readoperation, and therefore the Quad SPI read operation can provide fourtimes higher read thru-put. While Quad SPI read operation is used hereinfor explanation, the teachings herein are equally applicable to theother modes of operation, including but not limited to single SPI, dualSPI, Quad Peripheral Interface (“QPI”) and Double Transfer Rate (“DTR”)read modes. In the QPI protocol, the complete interface (command,address, and data-out) is done on 4-bit basis. In the DTR protocol, theoutput data is provided on both low-going and high-going CLK edge,rather than providing output data only on low-going CLK edge as inSingle Transfer Rate (“STR”) read mode operation.

Bad Block Management in the HPSF-NAND Memory

Because of the generally poor reliably of NAND memory cells relative toNOR memory cells, bad block management is employed. The NAND flashmemory array 340 includes three areas, the user-addressable area 342,the redundant block area 344, and the Look-Up Table (“LUT”) informationarea 346. The look-up table (“LUT”) register 327 stores a look-up table,which contains a mapping from logical block addresses (“LBA's”) tophysical block addresses (“PBA's”) for bad block management. Assume, forexample, that the NAND flash memory array 340 has several failed blocks.The failed blocks are mapped to good blocks so that the device mayremain in service. Illustratively, bad blocks are mapped first to unusedblocks in the redundant block area 344, and then to available blocks inthe user-addressable area 342. While any desired mapping scheme may beused, mapping first to the redundant block area maintains fulluser-addressable memory capacity for as long as possible. To enableeffective bad block management, the look-up table may be constructed inthe LUT register 327, which is directly accessible to the control logic330 and the mapping logic 328. Illustratively, the LUT register 327 isimplemented in a small and fast volatile memory such as SRAM memory,although it may be implemented in any manner desired and in one ormultiple parts. The LUT register 327 may be populated at chip power-upor upon reset by reading LBA and PBA data from the LUT information block346. Bad block management is described in greater detail in U.S. patentapplication Ser. No. 13/530,518 filed Jun. 22, 2012 (Michael et al.,On-Chip Bad Block Management for NAND Flash Memory), which hereby isincorporated herein in its entirety by reference thereto. Continuouspage read as described herein is referred to as “fast continuous pageread” in the aforementioned patent application.

FIG. 5 shows an illustrative process 400 for powering-up a serial NANDflash memory, including an automatic default page read which iscompatible with bad block mapping and which includes bad blockmanagement.

At power-up, the BUSY flag is initialized to 1 (block 402), a bufferflag BUF 347 (FIG. 4) is initialized to the default value of 0 (block404), the look-up-table is constructed in the LUT register 327 (block406) from information in the LUT information block 346, and a defaultpage address is loaded into the address register 326 (block 408).Replacement block processing is then performed, which involves searchingin the LUT register 327 to determine whether the block address portionof the address in the address register 326 matches any of the LBA's inthe LUT register 327 (block 410). This search may be performed quicklywithout significantly impacting read access time because the LUTregister 327 may be a small and fast SRAM that is on-chip and thereforelocally accessible by the control logic 330. If no match is found (block410—no), the LBA is used to read the default page into the page buffer338 (block 414). If a match is found (block 410—yes), a replaced badblock is indicated and the PBA of the replacement block is used insteadof the LBA in the address register 326 (block 412) to read the defaultpage (block 414). An ECC procedure is performed on the data in the pagebuffer 338 and the resulting ECC bits in the status register(s) 323 areset as appropriate (block 416). Upon completion of the ECC processing,the BUSY flag is reset to 0 (block 418).

ECC Processing in the HPSF-NAND Memory

To achieve fast code shadowing, the next command after power-up with thecontinuous read mode set (BUF=0) may be any of the common HPSF-NORmemory read commands 03h, 0Bh, 3Bh, 6Bh, BBh and EBh, although the FastRead Quad Output command 6Bh and the Fast Read Quad I/O command EBh aremost suitable since they can output four bits per clock for the highesttransfer rate. The continuous read mode read commands, which ignore anyaddress in the command address field, continuously read data seamlesslyacross page boundaries beginning with the page address contained in theaddress register 326.

Execution of the continuous read mode read commands may advantageouslybegin without any latency except for decoding of the command if the ECCprocedure is performed by the default page read operation duringpower-up. Accordingly, the illustrative power-up process 400 performs anECC procedure (block 416). While the ECC procedure may be performed onthe entire page buffer 338, if a ping-pong buffer arrangement is usedfor the page buffer 338 to improve the speed of the continuous read moderead, then the ECC procedure need be performed on only half of the pagebuffer 338 to also reduce the time needed to complete the default pageread.

FIG. 6 shows a data bus 510 and a NAND flash array 550, together with anillustrative implementation of a page buffer which includes a dataregister 540 that is organized in two portions which may be referred toas data register-0 (“DR-0”) and data register-1 (“DR-1”). The pagebuffer also includes a cache register 530 that is organized in twoportions which may be referred to as cache register-0 (“CR-0”) and cacheregister-1 (“CR-1”). Therefore, the page buffer may be thought of ashaving a portion which includes CR-0 and DR-0, and another portion whichincludes CR-1 and DR-1. In an illustrative example, the page buffer mayhave a capacity of 4K Bytes, divided into two equal portions of 2K Bytecapacity each. As such, the storage capacity for each of DR-0, DR-1,CR-0, and CR-1 is 1K Byte. DR may be used to refer to a full 2K Bytedata register (i.e. DR-0 plus DR-1) and CR may be used to refer to afull 2K Byte cache register (CR-0 plus CR-1). A different size of pagebuffer may be used and/or a division of the page buffer into more thantwo portions or into unequal portions may be done if desired. Two setsof control signals may be needed for two portions of the page buffer,unlike one set of control signals needed for an undivided page buffer.Furthermore, differences between the logical and physical NAND flasharray does not affect teachings herein. For example, the physical arraymay have two pages (even 2 KB page and odd 2 KB page) on one word line,so that a word line may be 4 KB of NAND bit cells. For clarity, thedescription and drawings herein are based upon the logical NAND flasharray. Furthermore, while the page buffer is organized into 2 portionsto support a continuous read operation, the change is transparent to theuser. The program operation may be done for standard page size of 2 KB,and standard read operation, e.g. command to read the page data fromcache after completing a page read operation, may be also done forstandard page size of 2 KB. As such the internal organization of thepage buffer into two portions is particularly suitable for thecontinuous page read operation, and even then is such that its internaldivision is transparent to the user. The page buffer for a NAND memoryarray may be suitably organized and operated to eliminate gaps anddiscontinuities in the output data during a continuous page read inaccordance with the techniques described in U.S. patent application Ser.No. 13/464,535 filed May 4, 2012 (Gupta et al., Method and Apparatus forReading NAND Flash Memory), which hereby is incorporated herein in itsentirety by reference thereto. Continuous page read as described hereinis referred to as “modified continuous page read” in the aforementionedpatent application.

FIG. 6 also illustratively shows an error correction circuit 520, whichlogically may be thought of as having a section ECC-0 which provideserror correction of the contents of the cache register portion CR-0, anda section ECC-1 which provides error correction of the contents of thecache register portion CR-1. Various ECC algorithms are suitable foruse, including, for example, Hamming ECC algorithm, BCH ECC algorithm,Reed-Solomon ECC algorithm, and others. While two logical ECC sectionsECC-0 and ECC-1 are shown as respectively interfacing with CR-0 and CR-1for clarity of explanation, either two physical ECC blocks or a singlephysical ECC block may be used to interface with both CR-0 and CR-1.

FIG. 6 shows operation of the data register 540 and cache register 530during the default page read operation at power-up. The address of thedefault page 552 is resident in the address register 326, and thedefault page is read from the NAND flash array 550 into both parts DR-0and DR-1 of the data register 540 (Operation A). Illustratively, 2 KB ofdata is transferred from page 552, and illustratively, the transfer mayproceed in one 2 KB transfer or in separate 1 KB transfers into DR-0 andDR-1, which may or may not be simultaneous. The time for a page readoperation (i.e. time to transfer page data from a NAND flash array tothe data register) is illustratively 20 μs, although the exact time mayvary depending on such design factors as the sensing circuit, type ofcell (single-level cell or multi-level cell), and the technology node(such as 50 nm or 35 nm). Next, the data in portion DR-0 of the dataregister 540 is transferred to portion CR-0 of the cache register 530(Operation B1), an ECC computation is performed on the data in portionCR-0 of the cache register 530 (Operation B2), and ECC-processed data isreturned to portion CR-0 of the cache register 530 (Operation B3). Thetime for the transfer from DR-0 to CR-0 (and DR-1 to CR-1 as well)varies depending on design choice, but typically ranges from about 1 μsto about 3 μs. The time required for the error correction circuit 520 tocomplete depends on the choice of ECC algorithm, the internal data bus,the on-chip timing oscillator period, and other design factors.Illustratively, assuming a physical design which uses one ECC circuitblock for both portions CR-0 and CR-1 of the cache register 530, andfurther assuming the time for CR-0 and CR-1 data to be sent out to be 20μs and the time for the DR-0 to CR-0 transfer and the DR-1 to CR-1transfer to be 2 μs, the error correction circuit may be designed tocomplete in 18 μs or less, thereby requiring 36 μs for both CR-0 andCR-1 if only a single circuit is used. The page read ends withoutclocking out data, although the data register 540 and the cache register530 are set up for a continuous read mode read command.

FIG. 7 shows an illustrative process 600 for a basic continuous readmode read which is compatible with bad block mapping and which includesbad block management, and which begins execution without any latencyexcept for decoding of the read command. FIG. 8 shows certain operationsinvolving the data register 540 and the cache register 530, which arereferred to in FIG. 7. Initial conditions for the read command are forthe default page starting address to be present in the address register326 and for ECC-processed data to be present in the page buffer.

Three essentially concurrent operations may then take place, namely thata first part of the page buffer, specifically portion CR-0 of the cacheregister 530, is output (block 630 and Bus Operation A1), ECC processingis performed on a second part of the page buffer, specifically portionCR-1 of the cache register 530 (block 632 and Buffer Operations A1, A2and A3), and the next page of data is read into the page buffer,specifically portions DR-0 and DR-1 of the data register 540 (block 634and Array Operation A2). The Bus Operation A1 and the Buffer OperationA1 start at about the same time (time 1) within time interval A, but thelatter is much small in duration. Similarly, the Array Operation A2 andthe Buffer Operation A2 start at about the same time (time 2 later thantime 1) within time interval A, but the latter is much small induration. The next page of data may be accessed by incrementing theaddress in the address register 326 with the on-chip address counter 329(FIG. 4), and then performing replacement block processing. Replacementblock processing need only be performed at the time of the first pageaccess and at each block boundary, although to avoid additional circuitcomplexity to detect such occurrences, replacement block processing maybe performed at each page access without harm.

Next, two essentially concurrent operations may then occur, namely thatthe second part of the page buffer, specifically portion CR-1 of thecache register 530, is output (block 640 and Bus Operation B1), and ECCprocessing is performed on the first part of the page buffer,specifically portion CR-0 of the cache register 530 (block 642 andBuffer Operations B1, B2 and B3). Bus Operation B1 tends to take themost time, so no additional time is needed provided the BufferOperations B1, B2 and B3 occur within the time of the Bus Operation B.The Bus Operation B1 and the Buffer Operation B1 start at about the sametime (time 1) within time interval B, but the latter is much small induration.

Since a full page of data is now output (Bus Operations A1 and B1) andthe ECC bits in the status register(s) 323 is (are) now set, a tentativebad block evaluation may be performed (block 650). Bad block evaluationis described in greater detail in U.S. patent application Ser. No.13/530,518 filed Jun. 22, 2012 (Michael et al., On-Chip Bad BlockManagement for NAND Flash Memory), which hereby is incorporated hereinin its entirety by reference thereto. Continuous page read as describedherein is referred to as “fast continuous page read” in theaforementioned patent application.

The continuous read mode process 600 repeats until terminated in anydesired manner. One illustrative technique is to stop the clock signalCLK followed by a low to high transition in /CS so that the continuousread is not resumed upon resumption of CLK. Alternatively, thecontinuous read mode may be designed to terminate upon assertion ofanother signal, after a predetermined or specified number of page reads,or in any other manner desired by the designer. Advantageously, since noadditional read commands are used to read the subsequent pages, commanddecode time is avoided. Advantageously, the ping-pong buffer techniqueallows ECC processing time and next page read time to be essentiallyhidden in the time used for continuous data output. Advantageously, theincorporation of a fast on-chip LUT register such as the LUT register327 (FIG. 4) which is locally accessible by control logic such as thecontrol logic 330 (FIG. 4), enables a continuous page read with badblock management from the NAND flash memory without significantlydelaying the page read time when a replacement block is encountered,thereby further helping to avoid any gaps or discontinuities across pageand block boundaries. Advantageously, the concurrent Bus Operation A1,the Page Operation A2 (delayed by the time needed for the BufferOperation A1), and the Buffer Operations A1, A2 and A3 may be designedto take about the same time, thereby optimizing time utilization. Thetime for transferring and outputting a half page of data on the data bus510 is illustratively about 20 μs assuming a clock frequency of 100 MHz,and the time for a page read operation is illustratively about 20 μs,although the time for any particular design may vary depending onvarious design considerations.

Advantageously, although the time required for a page read and ECCprocessing, illustratively about 40 μs, may be considered to be aninitial latency, it occurs during power-up and does not impact thecommand programming. In contrast, the continuous read mode reads have nolatency aside from the command decode time. Although the page readcommand may require a time to complete that may approach about 60 μs,the page read command is issued only once for a continuous read acrosssuccessive page boundaries.

The description of the invention including its applications andadvantages as set forth herein is illustrative and is not intended tolimit the scope of the invention, which is set forth in the claims.Variations and modifications of the embodiments disclosed herein arepossible, and practical alternatives to and equivalents of the variouselements of the embodiments would be understood to those of ordinaryskill in the art upon study of this patent document. For example,although many of the implementations described herein are for serialNAND memory, certain techniques described herein such as the power-upsequence, mode selection, and continuous data output across pageboundaries and from logically contiguous memory locations without waitintervals, may be used for parallel NAND memory. Moreover, specificvalues given herein are illustrative, and may be varied as desired.These and other variations and modifications of the embodimentsdisclosed herein, including of the alternatives and equivalents of thevarious elements of the embodiments, may be made without departing fromthe scope and spirit of the invention, including the invention as setforth in the following claims.

1. A method of operating a NAND flash memory device comprising:receiving a read command which corresponds to a high-performance serialflash NOR (“HPSF-NOR”) read command and is clock-compatible therewith;and providing, from the NAND flash memory device in response to the readcommand receiving step, a continuous data output across page boundariesand from logically contiguous memory locations without wait intervals.2. The method of claim 1 wherein the read command is one of a Readcommand 03h, a Fast Read command 0Bh, a Fast Read Dual Output command3Bh, a Fast Read Quad Output command 6Bh, a Fast Read Dual I/O commandBBh, or a Fast Read Quad I/O command EBh.
 3. The method of claim 1wherein the NAND flash memory device comprises a NAND flash memory arrayand a page buffer coupled to the NAND flash memory array, and whereinthe providing step comprises: reading a page of data from the NAND flashmemory array to the page buffer; ECC processing data in the page bufferto produce ECC processed data; and outputting ECC processed data fromthe page buffer; wherein time used for the page reading step and the ECCprocessing step is buried in time used for the data outputting step. 4.The method of claim 1 wherein the page reading step is performed usingbad block management.
 5. A method of operating a memory having a NANDflash memory array and a page buffer coupled to the NAND flash memoryarray, comprising: receiving a continuous read command comprising acommand code and a starting address; and providing, from the NAND flashmemory array via the page buffer and in response to the read commandreceiving step, a continuous data output across page boundaries and fromlogically contiguous memory locations without wait intervals; whereinthe providing step begins with column 00 of the page buffer regardlessof the starting address.
 6. The method of claim 5 wherein the readcommand is one of a Read command 03h, a Fast Read command 0Bh, a FastRead Dual Output command 3Bh, a Fast Read Quad Output command 6Bh, aFast Read Dual I/O command BBh, or a Fast Read Quad I/O command EBh. 7.The method of claim 1 wherein the NAND flash memory device comprises aNAND flash memory array and a page buffer coupled to the NAND flashmemory array, and further comprising: selecting a page of the NAND flashmemory array; storing data from the selected page in the page buffer;performing ECC computations on the data in the page buffer; outputtingthe data from the page buffer; and repeating the page selecting, datastoring, ECC computation performing, and data outputting steps so thatthe continuous data output is provided; wherein the page selecting stepinitially comprises selecting a default page in the NAND flash memoryarray, and subsequently comprises selecting successive sequential pagesof the NAND flash memory array; wherein the page selecting, datastoring, and ECC computation performing steps are initially performedautomatically during power-up of the flash memory, and are subsequentlyperformed in response to the read command; and wherein the dataoutputting step is performed in response to the read command.
 8. Themethod of claim 1 further comprising: selecting a default page of a NANDflash memory array of the NAND flash memory device during power-upthereof; storing data from the default page of the NAND flash memoryarray in a page buffer during power-up of the NAND flash memory;performing ECC computations on the data stored in the page buffer afterthe storing step; and receiving the read command.
 9. The method of claim1 wherein the NAND flash memory device comprises a NAND flash memoryarray and a page buffer associated with the NAND flash memory array,further comprising: setting the flash memory device in a continuous readmode or a buffer read mode; transferring a page of data from a defaultpage of the NAND flash memory array to the page buffer; after the pagedata transferring step, receiving the read command; and when the flashmemory device is in the continuous read mode, outputting the continuousdata output from the flash memory device in response to the read commandreceiving step.
 10. The method of claim 9 further comprising: ECCprocessing the default page of data in the page buffer to provide an ECCprocessed default page of data; wherein the read command is receivedafter the ECC processing step; wherein when the flash memory is in thecontinuous read mode, outputting the continuous data beginning with theECC processed default page of data in the page buffer.
 11. The method ofclaim 10 further comprising establishing the default page under amanufacturer's control.
 12. The method of claim 10 further comprisingestablishing the default page under a user's control.